The disclosure concerns an operating method for an electric motor vehicle, i.e. for a motor vehicle with at least one electrical machine but no internal combustion engine as the traction engine, and with an operating element for preselecting a coasting function, wherein, if the coasting function is preselected by the driver, under certain conditions there is an automatic change into a coasting mode, in which neither a drive torque nor a drag torque is exerted on the wheels of the vehicle. Moreover, the disclosure concerns an electric motor vehicle that is designed to carry out the method.
One such method is known from DE 10 2007 035 424 A1. It relates particularly to hybrid vehicles that comprise at least one electrical machine as a traction motor in addition to an internal combustion engine, yet an electrical machine is also intended to be able to be provided as a sole drive.
With such vehicles, energy recovery is usually carried out if the driver releases the gas pedal, wherein the electrical machine is operated as a generator for charging up an electrical energy reservoir, such as for example a traction battery.
However, with a hybrid vehicle the kinetic energy of the vehicle should not be recovered, i.e., stored in the energy reservoir, in all driving states with the gas pedal released. Rather, it is sometimes advantageous to provide free rolling for as long as possible, an operating state in which the traction motor is decoupled from the drive train or switched off. One such operating state, in which the wheels of the vehicle roll freely, is called coasting.
With the aforementioned known method, the driver has the option of deliberately selecting the coasting function by means of a dedicated operating element, for example on the gear selector lever, or by means of a pressure point gas pedal. In an alternative, the coasting function is associated with a released pedal set. If the driver has activated the coasting function, there is always an automatic change into the coasting mode if this has no effect on the driving dynamics. For reasons of energy efficiency, when coasting the clutch is disengaged and the internal combustion engine switched off and/or electrical machines are operated in the energy-saving state.
Directly coupled synchronous machines with permanent magnet excitation are usually used as electrical machines for electric motor vehicles. If no energy is delivered to such a synchronous machine while driving, the counter EMF produces a drag torque in the synchronous machine and hence an increased rolling resistance of the vehicle, which results in higher energy consumption and reduced range. The most efficient energy-saving mode is then to just deliver enough energy to the synchronous machine such that it exerts neither a positive nor a negative torque on the wheels. Yet even in this state some energy is consumed.
The object of the disclosure is to specify an operating method for an electric motor vehicle that enables a particularly economical and ergonomic coasting mode.
This object is achieved by a method and by a device with the features of the independent claims.
Advantageous developments of the disclosure are given in the dependent claims.
According to the disclosure, the predetermined conditions are first that the gas pedal is in a neutral position, i.e. in a position in which the driver is exerting no torque on the gas pedal, and secondly that the current speed of the vehicle is at or is exceeding a preset value.
This is based on the knowledge that with electric motor vehicles it has often proved to be inefficient to carry out a coasting mode at low vehicle speeds, and also that drivers are more likely to neither expect nor want a coasting mode in such cases. Rather, it has been shown that when using the disclosure drivers have to activate the coasting function and deactivate it again less often and therefore are able to and want to use the coasting function longer overall.
Moreover, with the disclosure the automatic change into the coasting mode is carried out by bringing the drive train of the vehicle into a state in which the at least one electrical machine is consuming no electrical energy.
The latter state is achieved in one embodiment of the disclosure owing to the electrical machine being an asynchronous machine and being changed into the coasting mode by first changing from torque control to revolution rate control of the electrical machine and then switching off the current feed to the electrical machine.
The aforementioned energy-saving is achieved in another embodiment of the disclosure owing to the electrical machine being an externally excited synchronous machine with a separate or external excitation winding and being changed into the coasting mode by first changing from torque control to revolution rate control of the electrical machine, then switching off the excitation current to the excitation winding, then bringing the revolution rate of the motor to zero and then switching off the current feed to the electrical machine.
Alternatively, the aforementioned energy-saving can be achieved owing to the electrical machine being a synchronous machine with permanent magnet excitation and an electrically operated clutch or a freewheel being provided in the drive train downstream of the electrical machine, as is known from DE 10 2009 054 872 A1. In this case, a change is made into the coasting mode by first changing from torque control to revolution rate control of the electrical machine, then possibly opening the clutch, then bringing the revolution rate of the motor to zero, and then switching off the current feed to the electrical machine.
The preset value for the minimum speed of the vehicle, at which a change into the coasting mode can be made, is in one embodiment at least approximately 50 or 60 km/h, as is typical for larger urban roads or arterial roads, is approximately 80 km/h in another embodiment, and is approximately 100 km/h in yet another embodiment, i.e. as is typical for highways or freeways.
The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.
The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.